In gas detection instruments employing an electrochemical gas sensor, a potentiostatic drive circuit is normally used to hold the sensor electrodes at the required potentials. These potentials may be equal or there may be a bias between them to drive the required electrochemical behaviour.
The current generated by a sensor between the sensing and counter electrodes (or between one of these electrodes and an additional electrode) is amplified by additional circuitry. If the sensor is provided with a diffusion limiting gas access barrier and the electrode activity is adequate to consume all of the target gas entering the cell, then the current generated is proportional to the concentration of target gas in the environment.
If, prior to installation in an instrument, the relevant electrodes of a gas sensor have not been maintained at the required relative potentials, for example (they have been allowed to drift to their open circuit potentials), there will be a significant delay in the sensor achieving its optimum operating condition. This could be due to chemical species forming on the electrode or in the electrolyte which would be consumed or reacted under normal operating conditions. For example, the sensor may exhibit an erroneous, high baseline reading indicating the presence of gas when in fact this output is merely a reflection of internal cell processes.
Such effects gradually decline with continued operation, but there can be a significant delay before the sensor is capable of performing to specification. This is termed a start-up or stabilization period. Such periods can be many hours in extreme cases, depending on the design of the sensor, the performance requirements and the period for which the electrodes have been allowed to drift.
For manufacturers who build and calibrate instruments (which may contain several different electrochemical sensors), such start-up delays can be a significant problem. They can lengthen the cycle time of the manufacturing process which in turn requires the instrument manufacturer to hold additional inventory and hence increase costs.
Sensors which normally operate under bias are usually shipped on a pcb provided with a potentiostat and circuit capable of maintaining the required potentials for a limited period (compatible with the interval between sensor manufacture and instrument integration). The cost of such boards is not negligible and they are usually recycled back to the sensor manufacturer. This is time consuming and undesirable, but still preferable to dealing with long start-up times. For the majority of sensors which operate at zero bias, such complexity is not required, and creating a shorting connection between the relevant electrodes is adequate to maintain the sensor in a state of readiness.
The prior art discloses attaching a spring (or similar) across sensor contact pins to create an electrical short. This is additional labor task which is detrimental to the productivity of sensor manufacture, and an equal labor task is necessary for the instrument manufacturer to remove the spring before installation. Many instrument manufacturers opt to not install a spring. The reduced labor cost is more valuable than a cycle time associated with waiting for the drive circuit to stabilize the electrodes.
Preferably shorting links will only be applied between the required electrodes. Otherwise there is a risk of maintaining an electrode at an undesirable potential which can itself be a source of startup delay. For example, it may be undesirable to connect the counter electrode to the shorted sensing electrode-reference electrode pair, as in many cases it has a different Open Circuit Voltage and would therefore disturb the natural equilibrium of the reference.
Thus, an approach which unselectively shorts all sensor pins is unlikely to be appropriate. This is one reason why simple foam pads of the type used to protect semiconductor components from electrostatic discharge are not an adequate solution to the problem, even for individually packaged sensors. Similarly, approaches where a metal foil-covered compliant pad is used to short all the pins has significant shortcomings for sensor users, even if the relatively small annular contact areas of the foils around the pins were felt to be adequate and reliable. An ESD bag would suffer from similar shortcomings.
For containers holding multiple sensors, the above noted problems are greater.
Other shortcomings are (a) that the contacts between the sensor pins are not robustly and continuously maintained and can depend on orientation; and (b) the resistivity of the materials used is not well matched to the requirements of the electrochemical processes. For example, many ESD foams exhibit resistances of the order of 100K ohms between sensor pins. ESD bags have too high a resistance and offer insufficient contact area with the pins.
Electrochemical toxic gas sensors are packaged in a number of configurations which include (but are not limited to) multiple sensors packaged in trays, and individual sensors packaged in pots. Following the end of the manufacturing process, after exposing the sensors to a target gas to ensure sensor performance to a respective specification, the sensors are placed in storage locations or packaging trays.
Using prior art methods, additional handling of the sensors is necessary to apply the shorting spring, or similar structure, with the possibility of double handling if sensors are stored in an un-shorted state, with springs added prior to shipping.